This invention relates to methods of microfabrication and nanofabrication. The invention also relates to methods of performing atomic force microscope imaging.
Lithographic methods are at the heart of modern day microfabrication, nanotechnology and molecular electronics. These methods often rely on patterning a resistive film, followed by a chemical etch of the substrate.
Dip pen technology, where ink on a sharp object is transported to a paper substrate by capillary forces, is approximately 4000 years old. Ewing, The Fountain Pen. A Collector""s Companion (Running Press Book Publishers, Philadelphia, 1997). It has been used extensively throughout history to transport molecules on macroscale dimensions.
By the present invention, these two related but, with regard to scale and transport mechanism, disparate concepts have been merged to create xe2x80x9cdip penxe2x80x9d nanolithography (DPN). DPN utilizes a scanning probe microscope (SPM) tip (e.g., an atomic force microscope (AFM) tip) as a xe2x80x9cnibxe2x80x9d or xe2x80x9cpen,xe2x80x9d a solid-state substrate (e.g., gold) as xe2x80x9cpaper,xe2x80x9d and molecules with a chemical affinity for the solid-state substrate as xe2x80x9cink.xe2x80x9d Capillary transport of molecules from the tip to the solid substrate is used in DPN to directly write patterns consisting of a relatively small collection of molecules in submicrometer dimensions.
DPN is not the only lithographic method that allows one to directly transport molecules to substrates of interest in a positive printing mode. For example, microcontact printing, which uses an elastomer stamp, can deposit patterns of thiol-functionalized molecules directly onto gold substrates. Xia et al., Angew. Chem. Int. Ed. Engl., 37:550 (1998); Kim et al., Nature, 376:581 (1995); Xia et al., Science, 273:347 (1996); Yan et al., J. Am. Chem. Soc., 120:6179 (1998); Kumar et al., J. Am. Chem. Soc., 114:9188 (1992). This method is a parallel technique to DPN, allowing one to deposit an entire pattern or series of patterns on a substrate of interest in one step. This is an advantage over a serial technique like DPN, unless one is trying to selectively place different types of molecules at specific sites within a particular type of nanostructure. In this regard, DPN complements microcontact printing and many other existing methods of micro- and nanofabrication.
There are also a variety of negative printing techniques that rely on scanning probe instruments, electron beams, or molecular beams to pattern substrates using self-assembling monolayers and other organic materials as resist layers (i.e., to remove material for subsequent processing or adsorption steps). Bottomley, Anal. Chem., 70:425R (1998); Nyffenegger et al., Chem. Rev., 97:1195 (1997); Berggren et al., Science, 269:1255 (1995); Sondag-Huethorst et al., Appl. Phys. Lett., 64:285 (1994); Schoer et al., Langmuir, 13:2323 (1997); Xu et al., Langmuir, 13:127 (1997); Perkins et al., Appl. Phys. Lett., 68:550 (1996); Carr et al., J. Vac. Sci. Technol. A, 15:1446 (1997); Lercel et al., Appl. Phys. Lett., 68:1504 (1996); Sugimura et al., J. Vac. Sci. Technol. A, 14:1223 (1996); Komeda et al., J. Vac. Sci. Technol. A, 16:1680 (1998); Muller et al., J. Vac. Sci. Technol. B, 13:2846 (1995); Kim et al., Science, 257:375 (1992). However, DPN can deliver relatively small amounts of a molecular substance to a substrate in a nanolithographic fashion that does not rely on a resist, a stamp, complicated processing methods, or sophisticated noncommercial instrumentation.
A problem that has plagued AFM since its invention is the narrow gap capillary formed between an AFM tip and sample when an experiment is conducted in air which condenses water from the ambient and significantly influences imaging experiments, especially those attempting to achieve nanometer or even angstrom resolution. Xu et al., J. Phys. Chem. B, 102:540 (1998); Binggeli et al., Appl. Phys. Lett, 65:415 (1994); Fujihira et al., Chem. Lett., 499 (1996); Piner et al., Langmuir, 13:6864 (1997). It has been shown that this is a dynamic problem, and water, depending upon relative humidity and substrate wetting properties, will either be transported from the substrate to the tip or vice versa. In the latter case, metastable, nanometer-length-scale patterns, could be formed from very thin layers of water deposited from the AFM tip (Piner et al., Langmuir, 13:6864 (1997)). The present invention shows that, when the transported molecules can anchor themselves to the substrate, stable surface structures are formed, resulting in a new type of nanolithography, DPN.
The present invention also overcomes the problems caused by the water condensation that occurs when performing AFM. In particular, it has been found that the resolution of AFM is improved considerably when the AFM tip is coated with certain hydrophobic compounds prior to performing AFM.
As noted above, the invention provides a method of lithography referred to as xe2x80x9cdip penxe2x80x9d nanolithography, or DPN. DPN is a direct-write, nanolithography technique by which molecules are delivered to a substrate of interest in a positive printing mode. DPN utilizes a solid substrate as the xe2x80x9cpaperxe2x80x9d and a scanning probe microscope (SPM) tip (e.g., an atomic force microscope (AFM) tip) as the xe2x80x9cpenxe2x80x9d. The tip is coated with a patterning compound (the xe2x80x9cinkxe2x80x9d), and the coated tip is contacted with the substrate so that the patterning compound is applied to the substrate to produce a desired pattern. The molecules of the patterning compound are delivered from the tip to the substrate by capillary transport. DPN is useful in the fabrication of a variety of microscale and nanoscale devices. The invention also provides substrates patterned by DPN and kits for performing DPN.
The invention further provides a method of performing AFM imaging in air. The method comprises coating an AFM tip with a hydrophobic compound. Then, AFM imaging is performed in air using the coated tip. The hydrophobic compound is selected so that AFM imaging performed using the coated AFM tip is improved compared to AFM imaging performed using an uncoated AFM tip. Finally, the invention provides AFM tips coated with the hydrophobic compounds.